1. Field of the Invention
The present invention relates to the field of transformers used to convert a voltage from the common mode into a differential mode and conversely. Such transformers are generally called “balun” transformers, for “balanced-unbalanced”.
2. Discussion
FIG. 1 very schematically shows an example of a balun transformer 1 in an application environment. This may be, for example, a portable phone transmission chain. This type of application currently uses balun-type devices since, on the antenna side, a single-end device is most often involved.
Transformer 1 is essentially formed of coupled inductive windings. A first winding is formed of two sections L1 and L2 in series and of same values, the junction point 2 of which forms the junction point (generally the ground) of differential mode inputs/outputs 3 and 4 of the transformer. A first end of winding L2 forms common mode input/output terminal 5 while the other end is in the air. A second winding is formed of two sections L3 and L4 of same values, in series, having their end terminals defining input/output terminals 3 and 4 on the differential mode side. Sections L1 and L2 are respectively coupled to sections L3 and L4. Junction point 6 of sections L3 and L4 receives a bias voltage Vp (D.C. component) of the transformer via an inductance L5, a capacitor C5 connecting terminal 6 to ground 2. Each terminal 3 and 4 is connected to a terminal TXA and TXB of a radiofrequency transceiver circuit 10 by a respective inductance L6 and L7. Further, terminals 3 and 4 are respectively connected to terminal 2 by capacitors C6 and C7. Inductances L6, L7 and capacitors C6, C7 form an impedance matching circuit between the differential balun impedance and the input impedance of circuit 10.
To make such a mode-switching transformer selective, a filter 11 connects input/output terminal 5 on the common mode side to transceiver antenna 12. Filter 11 is a band-pass filter which, especially in radiofrequency transceiver applications of mobile telephony type, must be selective.
To gain space, it has already been provided to form a mode-switching transformer of balun type by means of two respectively low-pass and high-pass filters formed based on capacitive and inductive elements.
FIG. 2 shows the electric diagram of such a conventional transformer 20. It shows a common mode input/output terminal 5 and two differential mode input/output terminals 3 and 4.
The actual transformer is formed of two inductances L21 and L22 and of four capacitors C21 to C24. Inductance L21 is in series with capacitor C21 between terminals 5 and 3, their junction point being grounded via capacitor C23. Capacitor C22 is in series with capacitor C24 between terminals 5 and 4, their junction point being grounded by inductance L22.
The setting of the D.C. component of the differential signal is provided on each of terminals 3 and 4 by voltage sources Vp, respectively 25 and 26. Two inductances L25 and L26 connect the positive electrodes of sources 25 and 26 to terminals 3 and 4, respectively. The function of inductances L25 and L26 is to serve as a trap to avoid for the dynamic signal crossing the mode-switching transformer to reach their power supplies. Capacitors C21 and C24 prevent the D.C. component from reaching terminal 5.
The branch formed of inductance L21 and of capacitor C23 forms a low-pass filter, while the branch formed of capacitor C22 and of inductance L22 forms a high-pass filter. The filters are sized to have a same cut-off frequency (frequency at which the attenuation corresponds to −3 dB). Further, each filter introduces a 90° phase shift in the common mode signal applied on terminal 5 with, however, an inverted sign according to the output terminal 3 or 4. Accordingly, a 180° phase shift is effectively obtained at the cut-off frequency between terminals 3 and 4.
FIG. 3 effectively illustrates the frequency response of gain G of the mode-switching transformer of FIG. 2 according to frequency f. In this example, the two curves LP and HP, respectively corresponding to the frequency responses of the high-pass and low-pass filters, cross at a 2.45-GHz frequency for which the attenuation is −3 dB. This frequency thus corresponds to the central frequency of the mode-switching transformer.
The imbalance between the two differential mode inputs/outputs as soon as it is departed from the 2.45-Gigahertz frequency limits the use range of such a balun. This imbalance is linked to the fact that the phase shift by more or less 90° is only maintained on a band which is limited, but adapted to most applications.
Another disadvantage is that two inductances L21 and L22 are necessary to form the filters. Further, two additional inductances L25 and L26 are necessary to cause the biasing.